/**
 * [trim description] 去掉首尾空格，这个方法执行效率是0毫秒级的
 * @return {[type]} [description]
 */
String.prototype.trim = function() {
    var str = this,
    str = str.replace(/^\s\s*/, ''),
    ws = /\s/,
    i = str.length;
    while (ws.test(str.charAt(--i)));
    return str.slice(0, i + 1);
}
